The number of bytes that the task has sent to the server in the request body.
- iOS 7.0+
- macOS 10.9+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
This byte count includes only the length of the request body itself, not the request headers.
To be notified when this value changes, implement the
URLSession: delegate method.